This invention relates to a method of coding a digital video signal representative of successive pictures by the use of correlation between the successive pictures. This invention relates also to a coding device for use in carrying out the method and to a decoding device for decoding an output signal of the coding device.
Each of the successive pictures is called a frame when the digital video signal is, for example, a television signal. On transmitting the television signal, an interframe predictive coding method is effective to achieve data compression of transmission data and to thereby reduce the amount of the transmission data as is known in the art. The interframe predictive coding method makes use of correlation between successive frames. That is to say, a difference of a picture element between two of the pictures is coded as a coded signal for transmission.
It is possible by the interframe predictive coding method to transmit a completely still picture and an approximately still picture with a high compressibility of the transmission data and to receive the transmission data to reproduce the picture with a high quality. This is because there is almost no difference between the picture elements of the completely and the approximately still pictures. The interframe predictive coding method is, however, defective in that the quality of the reproduced picture becomes low when the high compressibility is retained even for a large movement. This is because the difference becomes large as a result of a decrease in the correlation between successive pictures.
In order to remove the defect, an improved method is proposed. The improved method is called a motion-compensation method. In the improved method, each of the pictures is divided into a predetermined number of blocks of picture elements. A motion vector is detected which represents a movement of each block between two of the pictures. Prediction is carried out with a high accuracy by using the motion vector. With this structure, the difference of the picture element between two of the pictures becomes small on transmission of the picture which includes a large movement. It is therefore possible to transmit the picture including a large movement with a high compressibility of the transmission data and to receive the transmission data to reproduce the picture with a high quality. The improved method is disclosed, for example, in U.S. Pat. No. 4,307,420 issued to Yuichi Ninomiya et al and assigned to Nippon Hoste,cir/o/ Kuokai, a public television broadcasting association in Japan.
However, the improved method is still defective in that the amount of the transmission data becomes large when the movement has a high speed. This is because the difference becomes large between the picture elements of the pictures when the movement has a high speed.
In order to reduce the amount of the transmission data, subsampling of the coded signal is carried out. Such a subsampling method is disclosed by L. Chiariglione et al in IEEE Global Telecommunications Conference, CH1819.2 (1982), pages 810-814, under the title of "The Development of the European Videoteleconference Codec." According to Chiariglione et al, on-off control of the subsampling is carried out either at each scanning line or at a predetermined number of the lines. Inasmuch as the subsampling is carried out not only for a moving part of the picture but also for a still part of the picture, the picture is reproduced with a low quality for the still part.